PG,
I am sorry for what you are going through and i feel for you. Strange thing, i thought i had posted a reply earlier in the day but i think it got lost somewhere!
Anyway, my opinion is similar to what Nancy and TL have written in the above replies.
I know the fear that you feel because i was feeling the same a few weeks ago. When my difficult child was using just a few weeks ago, he was also hanging around members of a local gang. The stories that he has been telling me makes me scared for him even though he has been clean and away from these people for six weeks. But, one thing that gang members don't do is threaten one of their own even where money is concerned and so, if your difficult child was truly hanging around these people, they would probably not result to violence towards her. Most gang members/drug dealers will mostly go after a rival gang to protect their business and tuff. Also these dealers/gang are busy on the streets doing whatever they do so i doubt they would be able to keep tabs on difficult child the way she describes.
I know you tried to talk to the FBI, but if you have any connection at your local police station, maybe you could try to talk to somebody in the gang unit because they monitor all gang activities and they would know if difficult child is affiliated with these people. Additionally, if difficult child has a twitter account that's not private, check and see what her friends are writing. I found twitter more helpful than facebook because kids tend to write more and one friend can lead you to another and you can put one plus one together and know what's going on in the streets. I found that even these kids in gangs also maintain twitter and facebook and difficult child might have some as friends on both social sites.
Again, sorry for what you are going through and i hope everything cools down soon. ((hugs)).